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Weather Systems
• Stability
• Clouds
• Storms
• Applications
13
Stability
• warm areas on the ground develop lower pressure, as air is forced to rise
• expansion upward: thermal buoyancy of warm air "parcels, or thermals" in heavier
surrounding (ambient) air
• in rising, parcels change in character through reduction in pressure:
• Universal Gas Law: K = PV/T
• an increase in elevation accompanies a decrease in pressure
• i.e. the temperature in a rising parcel lapses (decreases) with height
• in dry air (without condensation) this is 1° per 100 m of rise
• the Dry Adiabatic Lapse Rate (DALR) is a constant.
• this is hypothetical only (in order to ignore any mixing effects):
• An adiabatic process such as this is one that is closed to exchanges of
mass or energy with its surroundings;
• it is independent of the ambient
• the rising air is seen as a closed “parcel”, subject only to laws of physics .
• the assumptions are unrealistic but will be relaxed later, as necessary to
add realism
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Stability
• graph paper was devised to show temperature (° T) by elevation (pressure: φ phi)
• T-phi diagram or tephigram depicts selected temperature “ascent paths” for air
parcels that may change elevation
• to predict the temperature in upwelling air (cooling) or sinking air (warming)
If air at 2000m is at 12°C and sinks
1000m it will have warmed by
10C° to 22°C.
If air at the surface is at 20°C and
rises to 1500m, it will have cooled
by 15C° to 5°C.
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Stability
• If the parcel is warmer than its surroundings it will float upward (instability)
• but if cooler it will sink downward (stability)
Measure the temperature T at any heights: measured ambient T, to compare with
expected dalr predictions of T in a displaced parcel:
• if ambient is less than predicted parcel temperature T1a < T1dalr there is
increased buoyancy and parcel ascends: unstable air
• if ambient is more than predicted parcel temperature T2a > T2dalr there is
decreased buoyancy and parcel descends: stable air
• If ambient equals predicted parcel temperature T3a = T3dalr there is no
buoyancy parcel remains stationary: neutral atmosphere
16
Stability
• an air parcel warmer than its surroundings will float upward (instability)
• an air parcel cooler than its surroundings will sink downward (stability)
2500
2000
Elevation
T1dalr
T1a
1500
T2dalr
T2a
1000
T3a
500
-10
-5
0
5
Air temperature
10
T3dalr
15
20
17
Stability
• surface heating raises T of the hypothetical surface "parcel" (dashed lines show
effect of surface heating from predawn to afternoon)
• as surface warms, higher adiabats are followed
• radio-soundings of ambient temperatures at heights produce an environmental
temperature curve (red) that is ambient to the hypothetical parcel (blue line);
• if a rising parcel cools at the DALR(blue line), and this is cooler than the
measured ambient temperature with height (red line), then the parcel
temperature remains below that of the ambient environment. Decreased
buoyancy in the parcel forces it to descend, inhibits vertical mixing: stable air.
18
Instability
• surface heating raises T of the hypothetical surface "parcel" (dashed lines from
predawn to afternoon)
• as surface warms, higher adiabats are followed
• radio-soundings of ambient temperatures at heights produce an environmental
temperature curve (red) that is ambient to the hypothetical parcel (blue line)
• if a rising parcel cools at the DALR (blue line), and this is warmer than the
measured ambient temperature with height (red line), then the parcel
temperature remains over that of the ambient environment. Increased buoyancy
in the parcel forces it to ascend, promotes vertical mixing: unstable air.
19
Clouds
• dalr assumes no moisture in the hypothetical parcel
• air always contains some moisture both as vapour and in liquid form as submicroscopic droplets
• steady state exists in which atmospheric water is constantly evaporating and
condensing. (and if near 0°C, freezing and thawing)
• temperature in the air determines the balance among these processes:
• as air cools, the evaporation rate decreases more rapidly than does the
condensation rate (less energy is available)
• a temperature (the dew point temperature) may be reached at which
evaporation is less than the condensation and the tiny droplets collide and
coalesce into larger (visible) cloud droplets
• further collisions of these are what forms actual raindrops that are heavy
enough to fall (Ice Crystal Process (Wegener -Bergeron - Findeison
Theory
20
Clouds
Maximum Specific Humidity by Temperature: dependency of maximum moisture
upon temperature (some texts show this as vapour pressure, or % by mass)
• at any given temperature, there is a steady
state of water vapour:
e.g. @20°C: 15 gm-3
@10°C: 9 gm-3
(gm-3)
• conversely, for every moisture content,
there is a temperature at which the air will
begin to display net condensation: Dew
Point temperature
e.g. for 15 gm-3: 20°C
Relative Humidity =
actual moisture content / maximum moisture
content for air at that temperature
e.g. air @20°C with 11 gm-3, RH=9/15 = 73%,
but if it cools to 10°C, RH=9/9=100%
T (°C)
21
Condensation
• a rising parcel of air will cool at the DALR only to the point that it reaches its
Dew Point, called the lifting condensation level (LCL)
•
if rising continues, the parcel will then cool at the Saturated Adiabatic Lapse
Rate (SALR), which is a variable lapse rate
•
the cooling rate is governed by the latent heat (QE ) released which
depends on absolute humidity
LCL
22
Clouds
As a fall day warms up the surface temperature rises the most :
• e.g. from 4°C to 20°C or more.
At each point in time the adiabats shift,
• e.g. by the time the surface is nearing 12°, 15°, 20°C as shown, adiabats
show more instability, buoyancy increases
23
Clouds
• Therefore clouds form at the elevation at
which the dew point is reached
• if the saturated adiabat soon crosses
the ambient environment temperature
sounding, thin layers of clouds form
• if the adiabat does not intersect the
ambient temperatures, and free
convection extends upward toward the
Tropopause, cloud tops will be high
(unrestricted – free convection)
The shaded areas are proportional to the
degree of instability or the energy of uplift
(turbulence)
If the air then descends, it will warm at the
SALR until clouds evaporate, then will
warm at the DALR
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Cloud classification and inference
Luke Howard (1802) a pharmacist and amateur meteorologist, identified three
distinct cloud forms:
• Cirrus (curl of hair) “Parallel, fibres”
• Stratus (layered) “A widely extended horizontal sheet, increasing from below.”
• Cumulus (heaped) “Convex heaps, increasing upward from a horizontal base.”
Nimbus is applied to clouds from which rain is falling: generally nimbostratus and
cumulonimbus.
Classification by height, and hybridization of forms:
• High clouds(to near tropopause; ice crystals): cirrus, cirrostratus, cirrocumulus
• Middle clouds: altostratus, altocumulus
• Low clouds: stratus, stratocumulus, cumulus
All clouds form by vertical uplift of air, with greater vertical development (towering
cumulus) associated with instability.
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Cloud forms:
Cirrus
Cumulus
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Cloud forms: Stratus
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Cloud forms: Cumulonimbus
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Mechanisms causing air to rise:
Orographic uplift: over higher land
• A Chinook (Föhn) may develop if descending air is warmed by a net release of
latent heat, to temperatures warmer than ascending moist air.
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Mechanisms causing air to rise:
• Convective Uplift
• especially on surfaces exposed to the sun and are dry, but also oceanic
cells are also significant
• if a stable atmosphere, uplift is subdued and these clouds are generally
ephemeral, evaporating as rapidly as they form
• if air cools to its dew point, clouds appear and motions are visible
• Virga are whisps or streaks of precipitation falling out of a cloud which
evaporate before reaching the ground
• individual convective cyclonic cells are usually short-lived but intense with
heavy showers capable of generating flash floods, strong winds and hail
posing risks for people and property
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Mechanisms causing air to rise:
Convective Uplift
• upward displacement as air is heated by the ground below
• if the air becomes unstable, thermals intensify, downdrafts may develop
adjacent to the strengthening cyclone
• some clouds dissipate, but certain ones may continue to grow, producing
turbulence and potentially damaging conditions
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• Tornadoes
• especially hazardous are the tornadoes which may develop
• energy of convection is accentuated when vertical vectors of up- and
down-drafts occur in conjunction with a squall line (a line of
thunderstorms, leading a cold front)
• these conditions permit the out-flowing down-drafts (also called
downbursts, microbursts or plough winds) which interact with other
storm cells, and produce the rotating motion of the tornado vortex
• initially around a horizontal axis, but is drawn to a vertical position in
forming the funnel
• especially violent when there is a strong contrast in air masses along
the front, with hot humid maritime tropical air being suddenly uplifted in
what are termed supercells
• description of their geographic distribution that has provided clues to
their origins, and perhaps will lead to better explanations, predictions
and options for what to do about tornadoes
• The most common documentation of tornadoes (~800 per year) is in the
US (http://www.nws.noaa.gov/om/brochures/tornado.shtml ): east of the
Rocky Mountains during the spring and summer months
• not common in Canada
• research continues on prediction
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Watching for tornadoes, and chasing them have become fashionable. Reporting
is therefore subject to bias, favouring the more densely populated areas, until
automated observation is universally available via satellites and/or networks of
Doppler radar. This radar identifies rotating motion in a cloud via the juxtaposition
of shortening (red) and lengthening (blue) reflected radar signals. (Christian
Doppler, 1840s: when objects move away from the observer, the radiation waves
they emanate tend to exhibit a lengthening of their frequency).
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Convective Storms
Midlatitude tornadoes
form
Tropical Cyclones (Hurricanes, Typhoons)
atmospheric vortices - extreme Lows;
from a single convective storm (i.e. a
thunderstorm or cumulonimbus)
diameter 100’s of meters
origin
large temperature gradient; upward
thrust is due to land-based solar heating
(water spouts occur over water); change
of wind speed and/or direction with
height creates a strong vertical shear of
the horizontal winds
atmospheric vortices - extreme Lows;
several to dozens of convective storms
duration minutes
winds
strongest tornadoes (Fujita Tornado
Damage Scale 4 and 5) have estimated
winds of 333 kph and higher
days
strongest hurricanes ( Saffir-Simpson
Hurricane Scale 4 and 5) have winds of 210
kph and higher; however landfall may spawn
tornadoes, since surface winds decelerate
sooner than those above Hurricane tracks
100’s of kilometers
near zero horizontal temperature gradient;
over equatorial oceans only , latent heat
release fuels further uplift/downdrafts in
spiralling rays around the calm eye
(descending air); die out over land as
moisture source is lost
(from NOAA, 2011: http://www.prh.noaa.gov/cphc/pages/FAQ/Hurricanes_vs_tornadoes.php)
http://www.aoml.noaa.gov/hrd/tcfaq/A1.html
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• Hurricanes
• tropical cyclones are the largest of convective storms
• hurricanes, typhoons as well as by other names
• among the most deadly of naturaltropical storms in Canada: wind, rain, storm
surge, and ocean waves hazards
• normally hundreds of kilometres (even 1000 km) across and, rotating around a
relatively calm “eye”, are winds that exceed 118 kmh -1.
• surrounded by several convective cells including tornadoes
• may persist for several days as they track across the Atlantic, Pacific or Indian
Oceans
• formation associated with:
• a very warm ocean surface (>26°C)
• in the tropics (beyond a few degrees of the equator, where there is no Coriolis
Force)
• away from Jet Stream interference
• a seasonal pattern of beginning around the time of the fall equinox in the
Atlantic
• initially a simple tropical thunderstorm
• may intensify in pressure gradient and therefore wind speed and expand in
size eventually reaching hurricane status as it migrates westward, then curls
north (and sometimes even eastward)
• patterns became much clearer once orbiting and geostationary satellites
provided visible-light images of cloud cover and thermal infrared data.
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• Hurricanes
• Many web sites offer video/animations of the development of hurricanes, both
from the past and as they develop:
http://www.atl.ec.gc.ca/weather/hurricane/bulletins_all_e.html
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• Hurricanes:
• See: http://www.nhc.noaa.gov/ current tropical storms/hurricanes
• http://fermi.jhuapl.edu/hurr/10/igor/igor_gs_overview.gif
• http://www.cawcr.gov.au/ research programs
for background theory and timely observations and predictions of hurricane activity as well
as advice on how to react to the warnings of hurricanes.
The energy of a hurricane is derived from the very high amounts of latent heat released as
huge amounts of water vapour condense, so as lower temperature s or limited moisture
supply is encountered, it is weakened and eventually dissipates. Monitoring of the winds
and updrafts in hurricanes is conducted by government research agencies primarily to
track their trajectories, enabling prediction of where they will contact coastlines (landfall).
In fact the US National Weather Service categorizes the severity of tropical cyclones and
hurricanes using the Saffir–Simpson scale, based in a part on its association with shoreline
risks).
Note: new GIS products: http://www.nhc.noaa.gov/gis/ current systems
http://www.srh.noaa.gov/gis/tropical/Models/kml/NHC_Model_Forecasts_AutoUpdate.k
ml and http://radar.weather.gov/GIS.html
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Hurricane-proof engineering and architectural solutions are being developed:
• sea walls, buildings on stilts, wind-resistant shutters and heavily anchored
roofs
• not widely accepted, especially where their costs are prohibitive
• Galveston, Texas which lies at or below the elevation of its sea wall after the
experiencing the most devastating natural disaster in US history (a 1900
hurricane that killed over 6000 people), has adopted many of these
procedures.
• in general, society is not willing to avoid the risk; in fact shorelines are
increasingly inhabited. Instead ,the response is to expect agencies to issue
warnings when it is necessary to evacuate, and in many situations to provide
safe shelter.
• Canada is not exempt from the risk of hurricanes, but most have weakened by
the time they reach here. The most devastating hurricane reach Canada was
Hurricane Hazel in 1954 which killed 81 people, 35 of whom were drowned
when the Humber River flooded their homes on Raymore Drive near Lawrence
and Weston Road.
• http://www.ec.gc.ca/ouragans-hurricanes/default.asp?lang=En&n=E1111740-1
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“On October 15-16th 1954, Hurricane Hazel dumped 210 millimetres of rain in the
Toronto region within 12 hours. Flooding was inevitable: steep slopes along rivers
and soil saturated by previous rainfall funnelled 90 per cent of the rain directly into
rivers and streams. Flows in the Humber River were four times greater than
previously recorded. Hurricane Hazel caused the most severe flood in the Toronto
area in recorded history.
Eighty-one people died and thousands of people were left homeless. Most of the
bridges on the west side of Toronto were destroyed or badly damaged, as were
many on the Don River. Many roads, parks, public utilities - even an entire street
of houses - were washed out. The damages were astronomical, reaching an
estimated $25 million in 1954 ($169.5 million in 2000 dollars).“
Because of its immediacy, many compelling accounts of the event have been
archived, for example, the recorded images of CBC television
(http://archives.cbc.ca/environment/extreme_weather/topics/77/ among others.
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• The ramifications of that event have been far reaching.
• Not only has its intensities of rainfall become the standard "event" for designing
bridges, culverts and dams, but flood plains now have the protection of law in
order to limit the public's exposure to flood hazards.
• Because of it, Conservation Authorities were empowered to manage runoff, to
identify, acquire and protect flood plains from development, to diminish the
effects of flooding, and have become ecosystem-based stewards of
watersheds and natural river functions.
• http://www.ec.gc.ca/ouragans-hurricanes/default.asp?lang=En&n=5C4829A9-1
(extratropical transition of Hurricane Hazel to a midlatitude frontal storm)
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• Frontal Uplift
• Air masses of contrasting properties (temperature and moisture)collide as warm
and cold waves develop along the Polar Front.
• When the Polar Front migrates northward (spring) and southward (fall), the
contrasts between warm and cold air masses and the succession of changes from
one air mass to the other creates abrupt changes in conditions.
43
Mechanisms causing air to rise
Frontal Uplift
• Along the front, centres of low pressure develop at the apex of wave-like
forms (Rossby Waves) along the front.
• The lead edge of the wave becomes a warm front and the trailing edge a front
http://www.theweathernetwork.com/index.php?product=weathermaps&pagecontent=weathermaps&maptype=sys
From the Weather Network
2011.09.29
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Frontal uplift
• Gradual uplift of the whole air column occurs along warm fronts
• rapid uplift of surface air at cold fronts
• Expressed as clouds:
45
When a warm front passes:
• the temperature increases as the warm air mass gradually replaces the
cold
• uplift is gradual as the warm air shears up over the cold air mass
• precipitation is steady and seldom intense, falling from overcast skies
(stratus clouds)
• in winter the precipitation may be in the form of freezing rain/drizzle
When a cold front passes:
• the temperature decreases suddenly as the cold air mass replaces the
warm
• precipitation is characterized as sudden intense showers and
thunderstorms of relatively short duration, falling from cumulus clouds
• if temperatures drop sufficiently in the cold sector, the rain may change
to snow squalls
• cold fronts tend to migrate more rapidly than the warm fronts
• the low centre acts as a hinge creating a wedge of warm air
• close to the centre, the pressure gradient steepens, winds intensify and
converge on the cyclone, and precipitation is more probable
• if the cold front overtakes the warm front, an occlusion forms, in which
the warm front only exists aloft, above colder air masses below
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• uplift initiates condensation
• dew and fog are not adiabatic, condensation alone without uplift; surface
cooling through:
• advection
• cold air drainage
• most commonly, overnight loss of longwave radiation
• continued condensation in rising air produces precipitation
Advection cooling
• advective cooling (warm air cooling by passing across a warm surface) can
also cause "lake effect" precipitation, such as the high snowfall (and rain)
downwind of water bodies that is common around the Great Lakes. See:
http://www.noaa.gov/features/02_monitoring/lakesnow.html
http://www.islandnet.com/~see/weather/elements/lkefsnw3.htm
• precipitation prediction involves not only expected condensation quantities,
but also rates of coalescence of cloud droplets into rain or snow (etc) of
sufficient mass that they fall
47
Applications
To assess risks – e.g:
• getting wet - outdoor activities
• getting hit by lightning (Each year in Canada, lightning kills an average of 16
people and causes more than 20 per cent of all forest fires
http://cwfis.cfs.nrcan.gc.ca/en_CA/fwmaps/fdr
http://www.ec.gc.ca/foudre-lightning/default.asp?lang=En&n=48337EAE-1
• getting hit by hail (While there has not been a single recorded death attributed to
hail in Canada, hailstorms probably cause the greatest economic losses of any
natural hazard in Canada in terms of property and crop damage
http://www.canhail.com/
http://www.icomm.ca/hazards/meteorological/hail.html
being late – transportation, communication
• getting too cold or too hot
• preventing or monitoring the spread of:
• wild-fire Lawson, O.B. Armitage, W.D. Hoskins 1996 Diurnal Variation in the
Fine Fuel Moisture Code http://www.for.gov.bc.ca/hfd/pubs/docs/Frr/Frr245.pdf
• pollution (affected by atmospheric stability)
.
Severe storms in Canada
• Types of storms
• Blizzards
• Hail
• Heavy rain
• Ice storms
• Lightning
• Thunderstorms
From
http://www.getprepared.gc.ca/knw/ris/streng.aspx#b6
http://ontario.hazards.ca/data/intro3-e.html
From: http://www.theweathernetwork.com/pollenfx/powcu
Note: The red bar indicates historically when pollen is
active for that source.
Data provided by: Aerobiology Research Laboratories
Physics, chemistry, history and geography of atmosphere all of considerable
significance to our understanding of it:
• processes of change are ongoing
• detected by observation (including now routine measurement) of
• temperature
• energy
• pressure
• winds
• gases (including water, emissions etc).
1. Descriptions are ongoing
2. Development of further explanations
3. Weather predictions are improving though imperfect
4. Management of individual, communal and societal responses continue.
Increasingly atmospheric concerns are become climatological…
References:
Ahrens, D. C., 1994: Meteorology Today. West Publishing, Co.,
Minneapolis-St. Paul.
Bluestein, H. B., 1993: Synoptic-Dynamic Meteorology in Midlatitudes.
Oxford University Press, New York. 2 vols.
Fujita, T. T. 1973. Tornadoes around the world. Weatherwise, 26: 56-62.
Fujita, T. T. and B. E. Smith. 1993. Aerial survey and photography of
tornado and microburst damage. In: The Tornado: Its Structure,
Dynamics, Prediction, and Hazards. C. Church, D. Burgess, C. Doswell
and R. Davies-Jones (Eds), Washington, D. C., Amer. Geoph. Union,
Geophysical Mono. 79: 479-493.
Environment Canada, Glossary Weather Related Terms
http://www.ns.ec.gc.ca/weather/glossary.html